(1) Field of the Invention
This invention relates to a solid-state image pickup device for use in a television camera etc. Particularly, it relates to a solid-state imaging device which has a photoelectric conversion layer, a plurality of switching elements and horizontal and vertical scanning circuits, all these constituents being provided in a major surface portion of a semiconductor body.
More specifically, it relates to a solid-state device which has a photoconductor and switching elements addressed by scanning circuits for reading out from the photoconductor optical information stored therein.
(2) Description of the Prior Art
Solid-state imaging devices are classified into two types; the type wherein optical signals are stored in photodiodes or MOS (Metal-Oxide-Semiconductor) capacitances formed on a semiconductor substrate, and the type wherein only scanning circuits (scanners) and groups of switches connected thereto are arrayed on a semiconductor substrate and they are overlaid with a photoconductive film for the function of photoelectric conversion. The former is such that scanning circuits, groups of switches for addressing and the photoelectric conversion elements are integrated, in general, on an identical plane and that the source junctions, for example, of insulated-gate field effect transistors (hereinbelow, termed "MOSTs") serving as the switches can be utilized also for photoelectric conversion elements, so it can be fabricated comparatively easily. It is therefore a solid-state imaging device which has been performed for long. Since, however, the elements taking charge of the various functions exist on the identical plane, the integration density is low. This is a problem which may be said fatal for the imaging devices requiring 500.times.500 or more picture elements, and has hindered enhancements in the resolution and photosensitivity of the solid-state imaging device. In contrast, the latter which has been devised relatively recently in order to solve the problem of the former is such that the integration density of picture elements (that is, the resolution) and the efficiency of receiving light become higher owing to the double-layer structure in which photoelectric conversion elements are formed on the scanning circuits and the groups of switches. It is therefore expected as the future solid-state imaging device. An example of the solid-state imaging device of this type is disclosed in Japanese Patent Application Laid-open Specification No. 51-10715 (filed July 5, 1974). FIGS. 1A and 1B show the construction and structure of the device for explaining the principle thereof, respectively. In FIG. 1A, numeral 1 designates a horizontal scanner for turning "on" and "off" horizontal switching elements 3, numeral 2 a vertical scanner for turning "on" and "off" vertical switching elements 4, numeral 5 a photoelectric conversion part utilizing a photoconductive film, numeral 6 a power supply input terminal for driving the photoelectric conversion parts 5, numeral 10 a signal output line, and letter R a resistor. FIG. 1B shows the sectional structure of the photoelectric conversion part and the switching elements in FIG. 1A. Symbol 5' indicates the photoconductive film, symbol 6' the power supply input terminal disposed through a transparent electrode 7, symbol 3' the horizontal switching MOST, symbol 4' the vertical switching MOST, and numeral 8 an insulating film. Numeral 11 denotes a semiconductor substrate, numerals 121 and 122 gate electrodes, and numeral 13 an electrode (of, for example, Al) which is held in ohmic contact with one end 91 of the switching MOST 4'. When the optical image of an object is focused on the photoconductive film through a lens, the resistance of the photoconductive film varies in accordance with the light intensity of the optical image, a signal voltage corresponding to the optical image appears at the one end 91 of the vertical switching MOST 4 (4'), and the variation is derived from an output terminal OUT through the signal output line 10 as a video signal.
The inventors fabricated the aforecited solid-state imaging device by the use of the P-channel MOS device fabricating technique which is the stablest in the MOS-IC technology, and evaluated its characteristics. More specifically, scanning circuits and groups of switches were constructed of P-channel MOSTs whose sources and drains were formed by diffusing P-type impurities into a Si substrate of the N-type conductivity, a Se-As-Te evaporation film which is used as the photoconductive film of an image pickup tube was disposed on the group of switches, and a transparent electrode was formed on the evaporated film (refer to U.S. Pat. Nos. 3,890,525 and 4,040,985). The Se-As-Te film has a component distribution in the direction perpendicular to the plane of the film, and contains Se 50% or more as its component ratio within the film. The Se-As-Te amorphous film can be evaporated at the normal temperature, and is desirable for the solid-state imaging device requiring a long lifetime because the film after the fabrication is stabler than the other photoconductive films even in the air. In this film (Se-As-Te amorphous film), however, the mobility of holes is greater than that of electrons, predominant conduction charges become the holes, and the main constituents of current are the holes. Therefore, the holes generated in the upper surface of the photoconductive film are mostly attracted to the transparent electrode 7 to which a minus voltage is applied, whereas the electrons scarcely reach the switch side electrode 13. As a result, the voltage variation on which the video signal is based does not arise in the switch side electrode 13, in other words, the photosensitivity is conspicuously low (since the P-channel MOST is operated by the minus voltage, the voltage variation owing to the storage of the electrons needs to appear in the switch side electrode). The sensitivity is the most important factor that determines the quality of the imaging device, and is a problem which must be solved by all means in order to realize household cameras being the main use of the solid-state imaging devices.